1. Field of the Invention
This invention relates generally to adjustable cam and cam followers, and particularly to engine valve mechanisms for internal combustion engines which are controlled to maximize the efficiency of the engine.
2. Description of the Prior Art
The present day automobile engines are generally designed to provide the maximum efficiency at a high load. Among the variable design parameters are the timing of the intake and exhaust valves. These valves are commonly operated from a cam shaft having fixed cams with the timing selected to optimize the performance of the engine at a selected load. It is also well known that the timing requirements differ for various loads and therefore a fixed valve timing system will give optimum performance at only one load. The valve timing also has an effect on the emission characteristics of the engine and the usual fixed timing mechanism does not provide the best emission characteristics.
Therefore, it is clear that if the opening and closing of the valve could be varied with the load on the engine, more efficient operation would be obtained and the fuel economy improved.
The optimum opening and closing points as well as the overlap between exhaust and intake valves vary with load and therefore it is desirable to provide valve operating mechanisms which can optimize these events.
The need for variable valve timing has been known for many years and a large number of attempts have been made to provide effective means for this purpose. One technique that has been explored involves the use of a cam shaft which may be xoved axially and which includes cams with axially varying lobes. Means are provided to move the cam shaft axially in accordance with the rpm or the load on the engine. The cam shaft lobe is then shaped to provide the optimum valve operating parameters for the corresponding load condition. The following U.S. patents describe various mechanisms utilizing an axially movable cam shaft:
U.S. Pat. No. 2,307,926 Griffith, et al
U.S. Pat. No. 3,618,573 Allen
U.S. Pat. No. 3,618,574 Miller
U.S. Pat. No. 3,633,554 Nakajima, et al
U.S. Pat. No. 3,638,624 O'Grady
U.S. Pat. No. 3,730,150 Godner, Jr.
U.S. Pat. No. 3,897,760 Hisserich
U.S. Pat. No. 3,915,129 Rust, et al
U.S. Pat. No. 3,945,355 Calvaic
U.S. Pat. No. 3,986,484 Dyer
U.S. Pat. No. 4,182,289 Nakajima
Although the axially sliding cam shaft is a workable approach to the problem, one serious disadvantage present in most of the above-listed patents is the necessity of having the cam follower move along the variable lobe of the cams. This generally necessitates a point contact at which point very high pressures occur during engine operation, resulting in rapid wear and eventual failure. For example in the Griffith, et al patent, a rolling ball is utilized which produces basically a point contact. O'Grady shows a valve having a rounded end which also makes a small contact with the load.
The patent to Rust teaches a ball contact with a flattened portion to attempt to provide a line of contact with the cam lobe face. The patents to Allen and Miller show a rocking follower to solve this problem.
Another problem relates to the necessity of providing a single cam for each valve that has a complex contour to obtain a desired advance in the lower rpm of the engine while maintaining the same valve overlap while attempting to provide a line contact between the lobes and the cam followers.